At present, most trays in domestic and foreign vapor-liquid mass transfer apparatus use arched downcomers, the overflow periphery of which is only the chord length and the arc of the arch does not function. Therefore, they can not offer great liquid capacity and is not suitable to the operation with high liquid-vapor ratios and under pressures. UCC in USA invented an MD tray (Multiple Downeomer Sieve Tray, U.S. Pat. No. 3,410,540) in 60s, which is a sieve tray using rectangular downcomers. All the four sides of the rectangle are overflow periphery and the overflow weir is 2-5 times longer than that of the arched downcomer, suitable to the operation with great liquid capacity and high liquid-vapor ratios. The downeomer is suspended in the vapor phase space and requires no receiving pan, and therefore the bubbling area can be increased by 10-15%. However, because the liquid flow on the MD tray has to turn for 90 degrees; its path is short and its distribution is less uniform, the tray efficiency is somewhat affected. When a tower is expanded and modified with MD trays, replacement of 3 by 4 or 2 by 3 is usually employed (TECHNOLOGY 1988, June 6, Oil & Gas Journal 54). In recent years, UOP Co. in USA has developed ECMD (enhanced Capacity Multiple Downcomer Sieve Tray)for increasing the width of the downeomer and enhancing the liquid treatment capacity, but no improvement is made in liquid flow distribution on the tray, see CHEMICAL ENGINEERING PROGRESS Vol. 66, No. 3.
U.S. Pat. No. 5,382,390 to Resetarits et al., proposes a multiple downcomer tray with notched directing slots. The directing slot is notched at the middle of the bubbling zone among the downcomers and oriented towards the overflow weir of the tray, i.e., the long side of the downcomer. The object of the slot is to push the liquid half way on the liquid flowing path across the tray and decrease the height of the froth and the entrainment of the droplet so as to enhance the vapor throughput. Resetarits thinks that, unlike the normal cross-flow tray, the multiple downcomer tray exhibits no phenomena of sluggish flow and liquid non-uniform residence time; the liquid flow distribution is already uniform. and thus there is no need to improve the liquid flow distribution. Therefore, notching multiple directing slots and determining their position and direction are all for pushing the liquid half way and enhancing the vapor throughout. The research for evaluating the uniformity of the liquid flow distribution by equivalent residence time distribution curves measured with an optical fiber instrument demonstrates that the liquid on the Resetarits' 390 multiple downcomer tray is injected in point form, and then turned for 90 degrees. Therefore, there exists an obvious non-uniform phenomenon (refer to FIG. 5(a)) for the liquid distribution.
The object of the present invention is therefore to notch directing slots and install directing baffles on the liquid receiving zone into which the liquid just enters so as to improve the initial distribution upon entering the tray, allow the liquid flow to turn for 90 degrees smoothly and distribute as uniformly as possible as shown in FIG. 5(b), and thus enhancing the separation efficiency.